Ion-channel-mimetic sensing of hydrophilic anions based on monolayers of a hydrogen bond-forming receptor

Ion-channel-mimetic sensing based on monolayers of a hydrogen bond- forming bis(thiourea) receptor was used to detect inorganic anions. Receptor monolayers were formed at the air-water interface and subsequently contacted with a highly oriented pyrolyric graphite electrode. Horizontal touch cyclic voltammetry was performed with subphase solutions containing various electroinactive analyte anions and [Fe(CN)₆]^4- as electroactive marker. Binding of analyte anions to the receptor monolayer was found to inhibit [Fe(CN)₆]^4- oxidation. The influences of the analyte anions on the cyclic voltammograms were largest for HPO₄^2- and decreased in the order of HPO₄^2- > F^- ≃ SO₄^2- > CH₃COO^- > Cl^-, whereas ion-selective electrodes (ISEs) containing the same receptor respond with a selectivity order of Cl^- > SO₄^2- > CH₃COO^- > H₂PO₄^-/HPO₄^2-. Because the bis(thiourea) receptor does not bind to all potentially hydrogen-bonding sites of most of these onions, it is apparent that several of the larger anions, and in particular phosphate and sulfate, are still substantially hydrated while being bound to the interfacial receptor layer. This distinct feature of interfacial molecular recognition seems to explain why selectivities of these ion-channel-mimetic sensors differ so strongly from the selectivities for the complete anion transfer from aqueous to organic phases, as represented by the ISE selectivity. The results in the present work suggest that ion-channel-mimetic sensors are particularly promising for the analysis of very hydrophilic, relatively large analytes, for which hosts that encapsulate the analyte and do not allow the analyte in this complex to be hydrated are difficult to synthesize.